Investigation of Exclusive Photoproduction of eta-rho0 and pi0-rho0 Meson Pairs at HERA

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Abstract

Exclusive photoproduction of $\eta-\rho^{0}$ and $\pin-\rho^{0}$ meson pairs is studied with the H1 detector at HERA. The analysis is based on data recorded in 1996-1997 and 1999-2000 data taking periods, corresponding to an integrated luminosity of $76.21\,\,{\tt pb}^{-1}$. %{/pb}$. The $\eta$ and $\pin$ mesons are identified via their photonic decays in the backward calorimeter SpaCal, and the $\rho^{0}$ meson via its dominant decay into $\pip\pim$ pairs in the central tracking system of the H1 detector. The scattered proton or its remnant is not measured but is separated by a large rapidity gap from the $\rho^{0}$ meson. \\ The cross section for photoproduction of $\eta-\rho^{0}$ meson pairs is measured at an average photon-proton center of mass energy of $W\approx208\,{\tt GeV}$ in the kinematical region defined by the rapidities of the $\eta$ and $\rho^{0}$ mesons in the laboratory system: $-3.5<Y^{lab}_{\eta}<-2$ and $-1.5<Y^{lab}_{\rho^{0}}<1.5$, photon virtualities $Q^{2}<0.01\,{\tt GeV}^{2}$ and inelasticity $0.3<y<0.65$. The cross section obtained is $\sigma^{*}(\gamma p\rightarrow\eta\rho^{0} X) = (3.5 \pm 1.4 (stat) \pm 1.3 (syst))\,\,\, {\tt nb}$. Due to the limited statistics no definitive statement is made to explain this process but the measured differential distributions are consistent with a contribution from Pomeron exchange.\\ In the analysis of photoproduced $\pin-\rho^{0}$ meson pairs no significant $\rho$ meson signal can be extracted. % the signal of the $\rho^{0}$ meson can not exctracted. Therefore only an upper limit of the cross section is given as $\sigma^{*}(\gp\to\pin \, \rho^{0} \, X) \, < \, 8.6 \,\,\,{\tt nb}$ at $95\,\%$ confidence level, measured at an average photon-proton center of mass energy of $W\approx180\,{\tt GeV}$ in the kinematical region defined by the rapidities of the $\pin$ meson and the $\pip\pim$ pair in the laboratory system: $-3.5<Y^{lab}_{\pin}<-2$ and $-1.5<Y^{lab}_{\pim\pip}<1.5$, photon virtualities $Q^{2}<0.01\,{\tt GeV}^{2}$, and inelasticity $0.3<y<0.65$.